Synthesis and 5-HT-3 receptor binding of 2- and 3-(halo)alkoxyl substituted (S)-N-(1-azabicyclo[2.2.2]oct-3-yl)-5-chlorobenzamides as potential radioligands.
In an effort to develop selective, high-affinity radioligands for the 5-HT-3 receptor, a series of homologues of 5-chloro-2,3-dimethoxy-N-(1-azabicyclo[2.2.2]oct-3-yl)benzamide (2b) was prepared in which individual methoxy groups were replaced by ethoxyl, (2-fluoroethoxyl), allyloxyl, propargyloxyl, or (3-iodoallyl)oxyl groups. Affinities for the 5-HT-3 receptor were determined by displacement of the binding of [125I]MIZAC (2a), a selective 5-HT-3 receptor antagonist radioligand, in rat brain homogenates. The 3-substituted homologues were more potent than the lead compound, 2b. The homologue having the largest 3-substituent, i.e., E-(S)-N-(1-azabicyclo[2.2.2]oct-3-yl)-5-chloro-3-(3-iodo-2-propenyl)oxy- 2-methoxybenzamide (3b, THIZAC), had one of the highest affinities, Ki 0.08 nM. The 2-substituted homologues were equipotent with 2b, having Ki 0.2-0.3 nM, regardless of the size of the substituent. The corresponding iodoallyl derivative, E-(S)-N-(1-azabicyclo[2.2.2]oct-3-yl)-5-chloro-2-(3-iodo-2-propenyl)oxy- 3-methoxybenzamide (4, LIZAC), displayed a Ki of 0.29 nM. Saturation binding of [125I]-4 gave a KD of 0.31 +/- 0.04 nM and a Bmax of 2.36 +/- 0.10 fmol/mg of entorhinal cortex. In vivo biodistribution of [125I]-4 in the rat brain showed increased accumulation in hippocampus relative to that in cerebellum. Both the high-affinity ligands [125I]-3b and [125I]-4 are potentially useful radioligands for studying the 5-HT-3 receptor.
Hewlett
W A
WA
Department of Psychiatry, Vanderbilt University School of Medicine, Nashville, TN 37232, USA.
Schmidt
D E
DE
Mason
N S
NS
Trivedi
B L
BL
Ebert
M H
MH
de Paulis
T
T
D000818:Animals; D001549:Benzamides / Q000138:chemical synthesis* / Q000493:pharmacokinetics / Q000494:pharmacology; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D002851:Chromatography, High Pressure Liquid; D007457:Iodine Radioisotopes; D007700:Kinetics; D008024:Ligands; D008297:Male; D011812:Quinuclidines / Q000138:chemical synthesis* / Q000493:pharmacokinetics / Q000494:pharmacology; D019275:Radiopharmaceuticals / Q000138:chemical synthesis* / Q000493:pharmacokinetics / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D017981:Receptors, Neurotransmitter / Q000187:drug effects / Q000378:metabolism; D011985:Receptors, Serotonin / Q000187:drug effects / Q000378:metabolism*; D014018:Tissue Distribution; D014055:Tomography, Emission-Computed
D001549:Benzamides; D007457:Iodine Radioisotopes; D008024:Ligands; D011812:Quinuclidines; D019275:Radiopharmaceuticals; D017981:Receptors, Neurotransmitter; D011985:Receptors, Serotonin